1. Field of the Invention
The present invention relates to an acceleration sensor.
2. Description of the Related Art
Japanese Patent No. 2780594 discloses an acceleration sensor employing a piezoelectric ceramic. This acceleration sensor includes a bimorph type detection element which is produced by laminating a pair piezoelectric devices formed of piezoelectric ceramics into a unitary body. The detection element is supported at both ends thereof and housed in a casing. When acceleration acts on the acceleration sensor, the detection element is deflected, and stress is generated in the defection element. The acceleration sensor detects acceleration, by detecting charge or voltage generated through piezoelectricity. The acceleration sensor is compact enough to be easily structured into a surface-mounting component (a chip component).
In the acceleration sensor working on this principle, a bias current flowing from a circuit of the sensor into the piezoelectric body is charged in a capacitor C of the piezoelectric body, and a resistor R is thus required to leak the bias current. The resistor R and the capacitor C form a filter, thereby leaving a direct current and a low frequency component below a cutoff frequency thereof undetected.
In a known acceleration sensor (disclosed in Japanese Unexamined Patent Application Publication No. 4-361165), two vibrators are mounted on a flex-type tuning fork support body. When acceleration acts on the vibrators, the vibrators mounted the tuning fork support structure are subject to tensile stress and compressive stress at central inertia portions (weight portions), and acceleration is detected from a frequency difference taking place between the two vibrators. This acceleration sensor detects a direct-current and low-frequency components.
Since the acceleration sensor thus constructed has the support body of tuning fork, the design of the structure becomes complex and bulky, and extensions of electrodes from the vibrators are also complex. It is therefore difficult to arrange this acceleration sensor in a miniature surface-mounting component (a chip component) which may directly be mounted on a printed circuit board.
The tuning fork vibrator is designed as a bimodal tuning fork vibrator to vibrate in a combination vibration mode in which a torsional vibration mode and a flexural vibration mode are combined, thereby reducing dependency of a bias frequency on temperature. This arrangement fails to fully eliminate the temperature dependency thereof.
Accordingly, it is an object of the present invention to provide a compact and high-gain acceleration sensor which is surface-mounted and insensitive to factors such as temperature changes other than acceleration.
The present invention in a first aspect relates to an acceleration sensor and includes a first resonator and a second resonator which resonate at independent frequencies and each of which includes a piezoelectric body and electrodes arranged on both main surfaces thereof and a first base plate and a second base plate, wherein a first unimorph type acceleration detection element includes the first resonator bonded to one surface of the first base plate, and a second unimorph type acceleration detection element includes the second resonator bonded to one surface of the second base plate, wherein each of the first and second unimorph type acceleration detection elements is fixed at one longitudinal end thereof or opposed longitudinal ends thereof such that the first resonator and the second resonator are substantially diametrically opposed to each other or are arranged to face each other to allow the first resonator and the second resonator to independently deflect in response to the application of acceleration, and wherein when the two acceleration detection elements are independently deflected in response to the application of acceleration, acceleration is detected by detecting a difference between frequency changes of the first resonator and the second resonator or a difference between impedance changes of the first resonator and the second resonator.
Since the acceleration detection element has a unimorph structure having the resonator and the base plate bonded to each other, compressive stress and tensile stress required by the resonator are effectively generated in response to deflection of the acceleration detection element taking place under acceleration. The pair of acceleration detection elements are constructed by connecting the pair of resonators in an end to end fashion or in a broadside to broadside fashion. When the one detection element detects tensile stress, the other detection element detects compressive stress. The resonance frequency of the tensile side resonator becomes lower while the resonance frequency of the compressive side resonator becomes higher. Acceleration is thus detected by detecting a difference between frequency changes of the two resonator or a difference between impedance changes of the two resonators. Since the frequency difference or the impedance difference is detected rather than individually picking up the frequency changes of the two resonators or the impedance changes of the two resonator, stresses commonly acting on the two resonators (a stress due to a temperature change, for example) cancel each other out. A high-gain acceleration sensor free from the effect of temperature changes results.
Preferably, a flexurally neutral plane of deflection in response to acceleration lies in the bonding surface between the first resonator and the first base plate or within the first base plate in the first element, and a flexurally neutral plane of deflection in response to acceleration lies in the bonding surface between the second resonator and the second base plate or within the second base plate in the second element. If the flexurally neutral plane lies in the resonator side, both compressive stress and tensile stress occur within the same resonator, resulting in a weaker output signal. To position the flexurally neutral plane in the bonding surface between the resonator and the base plate or within the base plate side, the flexural rigidity of the base plate is set to be not less than that of the resonator.
Preferably, each of the first and second resonators is a vibration mode element in which energy is trapped to the longitudinal center portion thereof, and gaps are respectively provided between the first base plate and the first resonator and between the second base plate and the second resonator, the gaps being larger in area than a trapped vibration region of each of the first and second resonators and smaller in area than a deflection region of each of the first and second resonators which deflects under acceleration.
Laminating the resonator and the base plate on the entire surfaces thereof is acceptable. However, it is noted that such an arrangement reduces performance of the resonator (such as Q and K factors) because the vibration of the resonator is restricted by the base plate.
If the resonator and the base plate are laminated together on the entire surfaces thereof, the acceleration detection element is efficient in generating stress in response to acceleration although the performance of the resonator slightly drops.
Preferably, longitudinal ends of the first and second acceleration detection elements are bonded to face each other with a spacer layer interposed therebetween, wherein the external surface of each of the first and second acceleration detection elements, facing in a direction in which acceleration is applied, is covered with a casing member, and each open surface which the first and second acceleration detection element and the casing member define is covered with a covering member, and wherein the electrodes arranged on the first and second resonators are respectively connected to external electrodes arranged on the surface of the covering member through internal electrodes arranged on the surface of the casing member.
In this arrangement, the acceleration detection element is fully enclosed the casing member and the covering member, and is thereby appropriate for use as a surface-mounting electronic component.
The present invention in a second aspect relates to an acceleration sensor and includes a first resonator and a second resonator, each resonator including a piezoelectric body and electrodes arranged on main surfaces thereof, and a single base plate, wherein the first resonator and the second resonator are respectively bonded to both sides of the base plate, wherein the acceleration detection element is fixed at one longitudinal end thereof or opposed longitudinal ends thereof such that the acceleration detection element deflects in response to acceleration applied in a direction in which the first and second resonators are laminated to each other, and wherein when the acceleration detection element is deflected in response to the application of acceleration, acceleration is detected by detecting a difference between frequency changes of the first resonator and the second resonator or a difference between impedance changes of the first resonator and the second resonator.
In contrast to the first aspect of the present invention in which the two unimorph type acceleration detection elements are employed, the present invention in the second aspect employs a bimorph type acceleration detection element which is constructed by bonding the resonators respectively to both sides of the single base plate. In this arrangement, a flexurally neutral plane (having zero stress) is set to lie within the base plate even if a relatively flexible material is used for the base plate, and the resonators arranged on both sides of the base plate effectively generate tensile stress and compressive stress. Acceleration is thus detected by differentially detecting the frequency changes of the two resonators or the impedance changes of the two resonators. The use of the single base plate reduces thickness dimension of the acceleration detection element, and the acceleration sensor is thus made compact.
Preferably, each of the first and second resonators is a vibration mode element in which energy is trapped to the longitudinal center portion thereof, and gaps are respectively provided between the base plate and the first resonator and between the base plate and the second resonator, the gaps being larger in area than a trapped vibration region of each of the first and second resonators and smaller in area than a deflection region of each of the first and second resonators which deflects in response to acceleration.
Preferably, the external surface of the acceleration detection element, facing in a direction in which acceleration is applied, is covered with a casing member, and each open surface which the acceleration detection element and the casing member define is covered with a covering member, and wherein the electrodes arranged on the first and second resonators are respectively connected to external electrodes arranged on the surface of the covering member via internal electrodes arranged on the surface of the casing member. In this arrangement, the acceleration detection element is fully enclosed the casing member and the covering member, and is thereby appropriate for use as a surface-mounting electronic component.
The acceleration sensor of the present invention uses two methods for differentially picking up the signals from the first resonator and the second resonator and for obtaining a signal proportional to acceleration acting on the acceleration detection elements. In one method, the first and second resonators are oscillated at different frequencies, a difference between the oscillated frequencies is detected, and a signal proportional to acceleration is determined from the frequency difference. In the other method, the first and second resonators are oscillated at the same frequency, one of a phase difference and an amplitude difference is obtained from a difference between electric impedances of the two resonators, and a signal proportional to acceleration is determined from one of the phase difference and the amplitude difference.
Acceleration is detected with high accuracy using either of the above two methods.